Good Day All! This is my first time on this forum and I would like to seek guidance and help.
Before we get started, I would like to explain that I have no electronics background or programming knowledge, so please don't be too hash on me if I seek a little clarification from you 
Requirements
The Idea: A Passive UHF RFID Tag with a temperature sensor.
Deployment: A running track with runners wearing a silicone band with the electronics inside.
Overall Requirements: Passive RFID ( No Batteries)
Range Requirements: 3 Meters(UHF?)
Temperature Sensing: +/- 0.5 Degree Celsius
Concurrent units: 30-80 different runners
Cost: As min as possible
Learning Curve: As short as possible
Inspiration
http://scanlime.org/2008/09/using-an-avr-as-an-rfid-tag/
Using a ATtiny85 to emulate a chip but with Low Frequency.
If I were to start out with the attiny85 as the main component of this project, would it be possible to:
Question 1: work this out with UHF?
Question 2: add a temperature sensor ?
Research
http://www.phaseivengr.com/wireless-technologies/wireless-technologies-overview/about-wireless-rfid-sensors/about-ultra-long-range-uhf-rfid-sensors/about-temperature-specialty-uhf-rfid-sensors/
http://www.farsens.com/media/files/PB-PYROS-0373-V04.pdf
http://www.rfid-cusp.org/papers/mtodd_thesis.pdf
Using an AVR as an RFID tag – scanlime
That was a VERY cool idea!
However, it would not be possible to use some of the tricks in that minimal RFID implementation at UHF. For example, the trick of using the RFID carrier wave as the clock signal would not work above the maximum clock frequency of the MPU, which at low voltages is about 4 MHz.
However, it would not be possible to use some of the tricks in that minimal RFID implementation at UHF. For example, the trick of using the RFID carrier wave as the clock signal would not work above the maximum clock frequency of the MPU, which at low voltages is about 4 MHz.
Yes it would you just need to demodulates it first.
Yes a good idea but I suggest it will take you three to five years to develop the skills to allow you to make it. Test equipment at UHF is not cheap either and working at those frequencies is a whole diffrent skill set that working with micro processors.
As to the implementation you are going to need a much lower current processor than the AtTiny85 with the ammount of power you can transmit over UHF.
Yes it would you just need to demodulates it first.
No, it would not, as the demodulated signal (demodulated with what?) would not be the carrier wave. I'll bet you didn't read the original article.
You can demodulates any signal with a number of methods you do not need a processor to do it. Yes you can do it with a fast processor but that is beside the point.